The present invention relates to a shift arrangement for motor vehicle transmissions and, more particularly, to such an arrangement for reducing operator shift effort when shifting from a two-wheel drive mode to a four-wheel drive mote under cold weather conditions.
In general, power transfer mechanisms are used in association with both manual and automatic transmissions for selectively directing power to the wheels of a motor vehicle upon shifting from the two-wheel drive mode to the four-wheel drive mode. It is known to use a gear synchronizer clutch arrangement in part-time transfer cases to provide "shift-on-the-fly" two-wheel drive to four-wheel drive "mode" shifting. One example of such an arrangement is shown and described in U.S. Pat. No. 4,677,873 issued Jul. 7, 1987 to Eastman which discloses a part-time transfer case having a gear synchronizer clutch unit incorporated as part of its "mode" shifting mechanism. Following speed synchronization, a fork mechanism shifts the synchronizer clutch sleeve into engagement with the external splines of a silent chain carrier for locking the transfer case in its four-wheel drive mode thereby delivering power via the drive chain to the vehicle's front output shaft.
In part-time four-wheel drive systems, various axle disconnect systems have been used for enabling the non-driven wheel(s) to rotate free of the remainder of its associated axle final drive assembly when the vehicle is operating in the two-wheel drive mode. One type of axle disconnect system includes use of manual locking hubs which the operator must unlock to permit the non-driven wheels to rotate free of its associated axle final drive assembly. Conventionally, "shift-on-the-fly" part-time transfer cases utilize automatic locking hubs having manual, electrical or fluid (i.e. vacuum) actuated front axle disconnect systems. These "automatic" systems connect the non-driven wheels to its final drive assembly for transferring drive torque from the transfer case to the non-driven wheels. Typically, connection is accomplished in response to a signal indicating that the vehicle operator has shifted into the four-wheel drive mode.
Prior patents disclose various means for enabling the non-driven wheel(s) to rotate free of the remainder of the front drive mechanism when in two-wheel drive mode. For example, U.S. Pat. No. 2,913,929 (Anderson) discloses a front axle design having complementary mechanism for cooperatively disconnecting both of the front wheels from the drive train to permit two-wheel drive operation. These complimentary mechanisms are actuated in common by a movable lever in the passenger compartment of the vehicle. The movable lever is connected to complementary clutch collars which are shiftable between two-wheel drive and four-wheel drive positions on the right and left axle shafts. Likewise, U.S. Pat. No. 2,770,150 (Culverwell) discloses a front axle design for a four-wheel drive vehicle which includes a mechanism for simultaneously disconnecting the right and left axle shafts from the center differential. In this type of system, disconnection of the front wheels takes place by shifting complementary clutch collars located within the front differential housing. Finally, reference may be had to U.S. Pat. No. 4,381,828 issued May 3, 1983 (Lunn et al) for details of a vacuum actuated axle disconnect system.
A common problem associated with part-time four-wheel drive drivetrains is that, during cold weather conditions, the vehicle operator must physically overcome excessive shift resistance when attempting to shift into the four-wheel drive mode upon starting the vehicle. The increased shift effort is due primarily to the high viscosity of the axle lubricant entrained within the front drive assembly and the inertial loading of the front prop shaft both of which must be overcome by the "mode" shift synchronizer clutch unit. However, this cold weather phenomenon is primarily temperature dependent in that excessive shift resistance is experienced upon attempting to shift into the four-wheel drive mode immediately after cold starting of the vehicle. Once the vehicle has been in operation for a period of time, the temperature of the front drive assembly lubricant increases and its viscosity decreases for restoring normal shifting operation of the synchronizer clutch unit.